1. Field of the Invention
The present invention is directed to load transducers, particularly those of the type that infer loading from changes in magnetic coupling.
2. Background Information
Transducers of loadxe2x80x94typically, force or pressurexe2x80x94sometimes infer loading from magnetic coupling. Typically, two members whose relative positions determine the magnetic coupling between them are resiliently mounted with respect to each other in such a manner that a load applied to the transducer causes a displacement between the membersxe2x80x94and thus a magnetic coupling between themxe2x80x94that is indicative of the amount of loading. One approach that has enjoyed some popularity employs a coil wound about a high-magnetic-permeability core. The core is one part of a magnetic circuit resiliently mounted for movement with respect to another high-permeability part. That movement changes the size of a low-magnetic-permeability portion of the magnetic path so that the path""s reluctance changes and thereby causes a measurable change in the coil""s inductance. Because of the resilient mounting, loading can be inferred from inductance.
Another approach to employing magnetic coupling for load measurement is exemplified by the arrangement described in U.S. Pat. No. 4,627,292 to Dekrone. A permanent magnet is resiliently mounted with respect to a core. The core is a ferromagnetic toroid about which two coils are so wound that the core provides magnetic coupling between them. Loading causes the magnet to move in a path, with which the magnet is aligned, that is generally perpendicular to the core""s axis. The magnet causes a degree of saturation in the corexe2x80x94and thus in the degree of magnetic coupling between the coilsxe2x80x94that depends on the magnet""s position. Loading can therefore be inferred by sensing the signal caused in one coil when the other coil is driven; i.e., the load determination can be based on the relationship between mutual inductance and load.
The Dekrone arrangement requires at least three-wire connections, and load transducers must often be disposed in locations where connection space is at a premium. So one may be inclined to adapt to load sensing the position-detection approach that U.S. Pat. No. 5,285,154 to Burreson describes. In one embodiment described in that patent, a permanent magnet is aligned with and displaced along a path that lies approximately on the axis of a coil wound about a ferromagnetic core. The magnet""s motion results in saturation changes in the core, and position is inferred from the coil""s inductance. This approach yields the sensitivity advantages of a saturation-type transducer, but it requires only a two-wire connection.
I have developed a partial-saturation-type load transducer that can be so configured as to achieve a higher sensitivity to load-caused displacement than similar-sized transducers of the Dekrone or Burreson type exhibit. In accordance with my invention, high-permeability material extends through most of the length of the path followed by most of the magnetic flux that the coil generates, but that path includes a relatively small gap, and the magnet is disposed in that gap. The transducer is so arranged that the magnet""s proximity to at least part of the path""s high-permeability segment responds resiliently to loading. This results in a change in the degree of core saturation, so the load can be inferred from the coil""s inductance. In contrast to the Burreson patent, this approach tends to afford relatively low reluctance, since the magnetic path consists largely of high-permeability material. But, in contrast with the Dekrone arrangement, the path includes a gap in which the permanent magnet can be disposed generally in alignment with the path so as to maximize the coupling between the magnet and the path. Employing my approach thus tends to afford enhanced transducer sensitivity to a given load-caused displacement.